Field of the Invention
The present invention relates to a photosensitive dry film that includes a curable resin layer on which pattern formation is feasible by exposure and development; and more particularly, related to a photosensitive dry film that can be suitably used for formation of a solder resist layer in particular for an IC package such as a solder resist of printed wiring board.
Background Art
In printed wiring boards used for electronic apparatuses, when electronic components are mounted on the printed wiring board, a solder resist layer is in general formed in an area other than connection holes on the board on which a circuit pattern is formed for the purpose of preventing soldering from adhering to unnecessary components and, at the same time, preventing a circuit conductor from being exposed to be corroded by oxidation or humidity.
Recently, printed wiring boards become more precise and denser due to miniaturization of electronic apparatuses. Associated with this, it is currently mainstream that a solder resist layer is formed by a so-called photosensitive solder resist, wherein a photosensitive resin ink is applied onto a base material; pattern formation is carried out by exposure and development; and then a resin after the pattern formation was is subjected to final curing by heating or light irradiation.
In addition, what has been also proposed is the use of a so-called photosensitive dry film, wherein the solder resist layer is formed without using the above liquid photosensitive resin ink, which allows a dry step following the ink application not to be required. Such a photosensitive dry film is in general composed of a laminate film stack in which a photosensitive resin layer is provided on a supporting film and a protective film is laminated on the photosensitive resin layer. When the film is used, a protective film is peeled and placed to a wiring board by thermocompression bonding. The resultant is subjected to exposure from above the supporting film; and thereafter the supporting film is peeled and subjected to development, thereafter forming a solder resist layer on which the pattern is formed. When compared with the case where the wet coating described above is employed for the film formation, the step of drying an ink can be omitted in cases where the solder resist layer is formed using the photosensitive dry film. On the top of that, because the dry film is thermocompressively bound to the circuit board, air bubbles are hard to be held between the board and the solder resist film and an improvement of filling up dents of the board surface is seen. In addition, because the exposure is carried out in a state where the solder resist layer is covered by the supporting film, the influence of oxygen to inhibit the curing is smaller; and the obtained solder resist layer exhibits a higher surface smoothness and a higher surface hardness, as compared with the case of the wet coating.
Meanwhile, it has been known that, by subjecting a solder resist layer to surface roughening, the solder adhesion resistance at the time of flow soldering improves; and thus a filler such as silica or talc has been conventionally added in a photosensitive resin composition. In addition, use of a photosensitive ink capable of forming a solder resist layer with low gloss without adding the filler has been proposed, which photosensitive ink is prepared by adding a specific photosensitive prepolymer in a photosensitive resin (for example, WO 2001/058977 and the like).
Further, Japanese Patent Application Laid-Open Publication No. 2012-141605 teaches that, by subjecting the surface of solder resist layer to surface roughening, wiring concealment improves and the gloss is kept lower, thereby obtaining good design properties. There is mentioned that, in cases where a solder resist layer is formed using a photosensitive dry film, the surface of the solder resist layer can be roughened by adjusting the surface roughness Ra of a supporting film to a range of 0.2 to 3 μm.